The use of radiation-sensitive silver halide emulsions for medical diagnostic imaging can be traced to Roentgen's discovery of X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913 the Eastman Kodak Company introduced its first product specifically intended to be exposed to X-radiation.
The first X-ray film contained a single radiation-sensitive silver halide emulsion layer coated on a transparent film support. However, the imaging efficiency was not comparable to that obtained by light exposure, since silver halide has a relatively limited ability to absorb X-radiation, which has a much higher energy level than visible light. As a result, patient exposures to X-radiation were quite high by modern standards.
Two improvements were introduced within 5 years of the initial X-ray film offering that are still used in forming most medical diagnostic images using X-radiation. First, the silver halide was coated on both the front and back sides of the film support (i.e., dual-coated) to double its absorption capacity, and, second, intensifying screens were mounted adjacent each emulsion layer. Each intensifying screen contains a phosphor that absorbs X-radiation and emits light. A dual-coated film-screen combination has about 20 times the imaging efficiency of the film alone.
Most medical diagnostic images are produced by dual-coated (e.g., Duplitized.TM.) film employed in combination with a pair of intensifying screens. A continuing problem with this arrangement has been that each intensifying screen emits light that is not only recorded by the silver halide emulsion on the adjacent side of the film support, but also by the silver halide emulsion on the opposite of the film support. This results in an reduction in image sharpness and is referred to as crossover.
The state-of-the-art of X-ray film construction through the 1970's is illustrated by Research Disclosure, Vol. 184, Item 18431, August 1979. Section V. Cross-Over Exposure Control specifically demonstrates techniques that have been proposed in the art to reduce transmission of light through the film support during exposure and thereby reduce crossover.
As radiologists began to generate large volumes of medical diagnostic images, the need arose for more rapid processing. The emergence of rapid access processing is illustrated by Barnes et al U.S. Pat. No. 3,545,971. Successful rapid access processing requires limiting the drying load--that is, the water ingested by the hydrophilic colloid layers, principally the silver halide emulsion layers, that must be evaporated to produce a dry image bearing element. One possible approach is to foreharden the film fully, thereby reducing swelling (water ingestion) during processing. Because silver image covering power (maximum density divided by the silver coating coverage) of silver halide medical diagnostic films was markedly reduced by forehardening of the films, it was the accepted practice not to foreharden the films fully, but to complete hardening of diagnostic films during rapid access processing by incorporating a pre-hardener, typically glutaraldehyde, in the developer.
Since silver halide emulsions require hydrophilic colloid for their preparation and full forehardening of non-tabular grain emulsion layers leads to marked reduction in silver covering power, reduction of the drying load placed on the rapid access processors has largely focused on limiting the hydrophilic colloid content of the medical diagnostic elements. However, when the hydrophilic colloid content of the emulsion layer falls too low, the problem of wet pressure sensitivity is encountered. Wet pressure sensitivity is the appearance of graininess produced by applying pressure to the wet emulsion during development. In rapid access processing the film passes over guide rolls, which are capable of applying sufficient pressure to the wet emulsion during development to reveal any wet pressure sensitivity, particularly if any of the guide rolls are in less than optimum adjustment.
Dickerson U.S. Pat. No. 4,414,304 (hereinafter referred to as Dickerson I) demonstrates full forehardening with low losses in covering power to be achievable with thin tabular grain emulsions.
Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 (hereinafter collectively referred to as Abbott et al) demonstrate that spectrally sensitized tabular grain emulsions are capable of reducing crossover to less than 20 percent. Subsequently, Dickerson et al U.S. Pat. Nos. 4,803,150 and 4,900,652 (hereinafter referred to as Dickerson et al I and II) demonstrated an ideal arrangement for essentially eliminating crossover by employing spectrally sensitized tabular grain emulsions in combination with front and back coatings that contain a particulate processing solution decolorizable dye interposed between the front and back emulsion layers and the support.
Dickerson et al II further addresses the advantage of accelerated rapid access processing attributable to the combination of a dual-coated format, tabular grain emulsions and controlled hydrophilic colloid coating coverages.
Luckey U.S. Pat. No. 3,859,527 proposed substituting for the prompt emitting phosphor in an intensifying screen a stimulable storage phosphor. This permits a retrievable medical diagnostic image to be captured and stored within the phosphor coating. The image is retrieved by subsequently stimulating emission from the phosphor layer and transferring the image information to storage within a digital computer for subsequent recreation of the image for viewing.
In recent years a number of alternative approaches to medical diagnostic imaging, particularly image acquisition, have become prominent. Medical diagnostic devices in addition to storage phosphor screens, including CAT scanners, magnetic resonance imagers (MRI), and ultrasound imagers allow information to be obtained and stored in digital form. Although digitally stored images can be viewed and manipulated on a cathode ray tube (CRT) monitor, a hard copy of the image is almost always needed.
The most common approach for creating a hard copy of a digitally stored image is to expose a radiation-sensitive silver halide film through a series of laterally offset exposures using a laser, a light emitting diode (LED) or a light bar (a linear series of independently addressable LED's). The image is recreated as a series of laterally offset pixels. Another approach is to use the image of a CRT monitor to expose a silver halide film.
Initially the radiation-sensitive silver halide films were essentially the same films used for radiographic imaging, except the silver halide emulsion is coated on only one side of the support, since exposing light is received entirely from the front side. Another adjustment was that finer silver halide grains were substituted to minimize noise (granularity). The advantages of the types of films conventionally used for medical diagnostic imaging to provide a hard copy of the digitally stored image are that medical imaging centers are already equipped to process silver halide medical diagnostic films and are familiar with their image characteristics.
A typical film, Kodak Ektascan HN.TM., for creating a hard copy of a digitally stored medical diagnostic image includes an emulsion layer coated on a clear or blue tinted polyester film support. The emulsion layer contains a red-sensitized silver iodobromide (2.5M % I, based on Ag) cubic grain (0.33 .mu.m ECD) emulsion coated at a silver coverage of 30 mg/dm.sup.2. A conventional gelatin overcoat is coated over the emulsion layer. The total hydrophilic colloid coating coverage on the front side of the support is 44.1 mg/dm.sup.2. On the back side of the support a pelloid layer containing a red-absorbing antihalation dye is coated. A gelatin interlayer, used as a hardener incorporation site, overlies the pelloid layer, and a gelatin overcoat containing an antistat overlies the interlayer. Developed silver is relied upon to provide the infrared density required to activate processor sensors. No dye is introduced for the purpose of increasing infrared absorption.
Typically silver halide diagnostic films, including the film described above, is processed in a rapid access processor in 90 seconds or less. For example, the Kodak X-OMAT M6A-N.TM. rapid access processor employs the following processing cycle:
______________________________________ Development 24 seconds at 35.degree. C. Fixing 20 seconds at 35.degree. C. Washing 20 seconds at 35.degree. C. Drying 20 seconds at 65.degree. C. ______________________________________
with up to 6 seconds being taken up in film transport between processing steps.
A typical developer (hereinafter referred to as Developer A) exhibits the following composition:
______________________________________ Hydroquinone 30 g Phenidone a 1.5 g KOH 21 g NaHCO.sub.3 7.5 g K.sub.2 SO.sub.3 44.2 g Na.sub.2 S.sub.2 O.sub.3 12.6 g NaBr 35.0 g 5-Methylbenzotriazole 0.06 g Glutaraldehyde 4.9 g Water to 1 liter/pH 10.0 ______________________________________
A typical fixer exhibits the following composition:
______________________________________ Sodium thiosulfate, 60% 260.0 g Sodium bisulfite 180.0 g Boric acid 25.0 g Acetic acid 10.0 g Water to 1 liter/pH 3.9-4.5 ______________________________________
Dickerson et al U.S. Pat. No. 5,637,447 discloses a radiation-sensitive film for reproducing digitally stored medical diagnostic images through a series of laterally offset exposures by a controlled radiation source followed by processing in 90 seconds or less including development, fixing and drying is disclosed. The film exhibits an average contrast in the range of from 1.5 to 2.0, measured over a density above fog of from 0.25 to 2.0. An emulsion is provided on the front side of the support. The emulsion contains silver bromochloride grains (a) containing at least 10 mole percent bromide, based on silver, (b) having a mean equivalent circular diameter of less than 0.40 .mu.m, (c) exhibiting an average aspect ratio of less than 1.3, and (d) coated at a silver coverage of less than 40 mg/dm.sup.2. Adsorbed to the surfaces of the silver bromochloride grains is at least one spectral sensitizing dye having an absorption half peak bandwidth in the spectral region of exposure by the controlled exposure source. The film also contains an infrared opacifying dye capable of reducing specular transmission through the film before, during and after processing to less than 50 percent, measured at a wavelength within the spectral region of from 850 to 1100 nm.